1. Field of the Invention
The present invention relates to a filter circuit that is usable in an electric vehicle and a hybrid vehicle, for example.
2. Description of the Related Art
In an electric vehicle and a hybrid vehicle, as a power source that is usable for driving the vehicle, a secondary battery such as a lithium ion battery is used, for example. Furthermore, because the voltage is low with only a single battery, a battery pack circuit that is configured with a number of battery cells, for example, of eight or more being connected in series is used.
When such a battery pack circuit is employed, whether each of the battery cells is operating properly along with operations such as charging and discharging has to be monitored at all times. In general, the voltage between the terminals of each battery cell is measured, and by the measured voltage, the state of each battery cell is comprehended.
The actual voltage of each battery cell, however, is affected by complex fluctuations in current along with the charging and discharging, the intrusion of noise from outside, and others. That is, when measuring the DC voltage of each battery cell is attempted, there is a possibility of the occurrence of a serious measurement error, by the influence of noise of AC or high frequencies.
Consequently, in a battery-voltage measurement circuit disclosed in Japanese Patent Application Laid-open No. 2003-282158, an RC filter unit is connected between each output terminal of a battery pack circuit and an input terminal of a voltage measurement circuit, for example. That is, the voltage is measured after removing the noise components of AC and high-frequencies by using a low-pass filter configured with a resistor (R) and a capacitor (C).
In the RC filter unit in Japanese Patent Application Laid-open No. 2003-282158, however, because a capacitor is connected between the terminals of each of the battery cells, there is a possibility of not being able to remove the noise adequately. That is, the capability of removing the noise that arises between the terminal of each battery cell and the ground is low.
For example, when a capacitor of a filter is connected between the terminal of each battery cell and the ground, the capability of removing noise can be improved. In that case, however, the voltage applied between the terminals of the capacitor becomes high, and thus it is necessary to increase the withstand voltage of each capacitor. In this case, it is further conceivable that, due to a DC bias voltage of the capacitor, the capability of the filter may be deteriorated and an adverse effect may appear in the measurement of voltage.
For example, it is conceivable to employ, as a component of the filter, a ceramic capacitor that is relatively small, high in withstand voltage, and inexpensive. The ceramic capacitor, however, has a tendency of the capacitance to fluctuate depending on the applied DC bias voltage. When the capacitance fluctuates, the cutoff frequency of the filter is varied, and thereby the noise in low frequencies may not be removed adequately. Furthermore, when the cutoff frequency of the filter is conversely too low, the effect of the filter appears in also fast fluctuations in voltage of the battery cell itself other than the noise, and thus it is conceivable that the capability of monitoring the voltage may be deteriorated and that the capacitor may increase in size more than necessary.